1. Field of the Invention
The present invention generally relates to a black and white, mono-color or full-color image forming apparatus, and more particularly, it relates to a full-color image forming apparatus such as a full-color electrophotographic copying machine, printer and the like, wherein an image is obtained by transferring and superimposing visualized images or toner images formed on an image bearing member by an electrophotographic technique or an electrostatic recording technique on a transfer sheet carried by a transfer sheet bearing member by the use of the transfer electric field.
2. Related Background Art
In the past, various kinds of full-color image forming apparatuses have been proposed. In such image forming apparatuses, an electrostatic latent image is formed on a photosensitive drum acting as an image bearing member and then is visualized as a toner image with yellow toner (having an average volume particle diameter of 11.0-14.0 .mu.m) contained in one of a plurality of developing devices. The yellow toner image is transferred onto a transfer sheet carried by a transfer drum acting as a transfer sheet bearing member at a transfer station. The transfer drum includes a pair of cylinders arranged on respective ends, a connecting portion connecting these cylinders to each other, and a dielectric sheet (transfer sheet bearing member) for covering an opening obtained by cutting off the drum portion while retaining the cylinders and the connecting portion. In order to transfer the image onto the transfer sheet at the transfer station, a transferring corona discharger (transfer member) comprising a wire electrode and a shield electrode surrounding the wire electrode is disposed within the transfer drum. Magenta, cyan and black toner images are formed on the image bearing members in the same manner as the yellow toner image, and the same transfer sheet is brought to the transfer station four times to transfer these four color toner images thereonto. After these four color toner images have been transferred onto the transfer sheet, the sheet is separated from the transfer drum. Thereafter, the transfer sheet is sent to a fixing device, where the four color toner images are melted and combined to form a full-color image fixed to the transfer sheet.
In the above-mentioned conventional technique, although the image obtained by the full-color electrophotographic copying machine which has been practically used provides improved image quality regarding the color reproductibity and the color tone reproductibity, it is hard to say that the image is closely reproduced from the color original. Further, unlike a television image, photo-image or color printed matter image, since the reproduced or copied image is not immediately compared with the actual object, the images obtained by the full-color electrophotographic copying machines which have been practically used are not necessarily satisfactory for persons who are accustomed to seeing images which are made more beautiful than the actual images.
Although techniques in which toner having a smaller particle diameter is used for improving the image quality have been developed, if the toner having a smaller particle diameter is used in the above-mentioned conventional image forming apparatus, since the corona discharger is used to transfer the toner image onto the transfer sheet, the following problems arise.
Generally, as the diameter of the toner particle is decreased, the BET specific surface area measured by the nitrogen gas adsorption technique is increased, so that the friction charging feature of the toner regarding the friction chargeable members such as carriers in developer, a developing sleeve (developer bearing member) and a regulating member for regulating an amount of the developer on the developing sleeve is enhanced, with the result that an amount of the frictionally charged toner is increased, thus worsening the movement of the toner. Further, when the specific surface area is increased, an amount of moisture adsorbed on the surface is increased, so that the change rate of the friction charging feature of the toner due to the environmental change is also increased, thus increasing the amount of the frictionally charged toner, particularly under the low humidity circumstance.
By way of an example, conventionally, the toner that the distribution of toner particle sizes corresponds to the average volume particle diameter of 11.0-14.0 .mu.m has been used. However, as mentioned above, when the toner having the smaller particle diameter, for example, the toner having the average volume particle diameter of 8.0-4.0 .mu.m and the greater friction charging amount (particularly, under the low humidity circumstance) thereby worsening the movement of the toner is used, the toner is strongly adhered to an electrostatic latent image electrostatically (for example, by the Coulomb force or reflection force), with the result that a stronger transfer electric field or transfer current must be supplied to transfer the toner image onto the transfer sheet. That is to say, conventionally, although high voltage of 6-8 KV was applied from a power source to the wire electrode of the transferring corona discharger so that a part of the total corona discharging current (about 10-30%), i.e., current of a few .mu.A-20 .mu.A is applied toward the photosensitive drum as the transfer current, if the toner having the smaller particle diameter is used as mentioned above, a further stronger transfer current is required.
In view of the construction of the transferring corona discharger, since the transfer current must be applied toward the photosensitive drum via the dielectric shield, the impedance of the corona discharge toward the photosensitive drum becomes very high, with the result that the corona discharge is effected actively between the corona discharger and the shield which is earthed and which has the lower discharge impedance. Consequently, almost all of the total corona discharging current will flow toward the shield. Accordingly, even if the current to be applied is increased to obtain a stronger transfer current, the increment of the transfer current toward the photosensitive drum will be limited to a certain extent. Thus, it is required to provide not only a power source having a greater voltage capacity and current capacity but also to use insulators having greater pressure resistance for the wiring, thereby making the apparatus bulky and expensive.
Further, since transferring corona discharger is used, the corona discharge is generated to produce ozone and nitrogen oxides which cause a bad influence upon the photosensitive drum. Further, in the case where the corona discharge has the positive polarity (i.e., the toner, has the negative polarity), if the corona application voltage is too high, the streamer discharge will occur immediately between the discharging wire and the shield having the lower impedance, thus causing irregularity in the discharge which leads to distortion of the transferred image.
Incidentally, if the transfer sheet is contacted to the dielectric sheet by using a conductive metal sheet or a conductive roller (referred to as "conductive contacting member" hereinafter) to which the voltage is applied, in place of the corona discharger, the above problem can be eliminated. That is to say, in this case, unlike to the corona discharger, almost all of the discharging current does not flow toward the shield, and thus, the transfer of the toner onto the transfer sheet can be achieved with the low potential, low current and high efficiency without producing ozone. However, if such conductive contacting member is used, since the transfer sheet is engaged by such member with straight line contact, the sheet is also contacted by the dielectric sheet with straight line or surface contact. In this condition, although it appears macroscopically that the transfer sheet is uniformly contacted with the dielectric sheet, microscopically, an irregular contacting condition between the transfer sheet and the dielectric sheet along the longitudinal direction of the photosensitive drum arises.
Accordingly, if a low voltage is applied to the conductive contacting member, the transfer charging current will be insufficient at weaker contacting areas (between the transfer sheet and the dielectric sheet), with the result that there is a difference in the transferring efficiency along the longitudinal direction of the photosensitive drum. Consequently, in the toner image transferred to the transfer sheet, there arises an irregularity in the toner density along a direction parallel to an advancing direction of the transfer sheet. On the other hand, if a high voltage is applied to the conductive contacting member, the space discharge will be generated at the weaker contacting areas where there are microscopic spaces between the dielectric sheet and the conductive contacting member, thus creating an irregularity in the toner density, discharge unevenness and/or transfer void in the transferred image. Accordingly, the proper voltage applicating condition is limited to a narrower range.
Further, even within the proper voltage applicating condition, the difference in the contacting condition between the conductive contacting member and the dielectric sheet leads in a difference in the transferring efficiency. Thus, even if the mono-color image has a slight irregularity in the toner densiy, when such images are superimposed on the single transfer sheet to form the multi-color toner image by using the multi-color image forming apparatus, the irregularity in the toner density will be noticeable.
In order to eliminate or reduce the difference in the contacting condition between the conductive contacting member and the dielectric sheet, the contacting pressure between the dielectric sheet and the conductive contacting member may be increased. However, if the contacting pressure is too high, the dynamic friction force between the conductive contacting member and the dielectric sheet will be increased to increase the load acting on the dielectric sheet, with the result that the dielectric sheet is displaced or distorted, thus displacing or distorting the transfer sheet carried by the dielectric sheet. Consequently, when this technique is applied to the multi-color image forming apparatus wherein various color toner images are transferred and superimposed on the single transfer sheet, there arises a discrepancy in colors and/or an irregularity in the color density in the multi-color image. Further, during the transferring operation, since the toner is strongly urged against the photosensitive drum, the transferring efficiency is decreased, and the service lives of the dielectric sheet and of the conductive contacting member are shortened, resulting in the frequent replacement of them, which increases the cost.
Further, when the transfer drum as mentioned above is used, if the contacting pressure between the dielectric sheet and the conductive contacting member is high, whenever members thicker than the dielectric sheet (such as the connecting portion for supporting the cylinders on both ends of the drum, and grippers for gripping a leading end of the transfer sheet) pass through the contacting area, a strong impact occurs at the leading and trailing ends of such members, thus scattering the toner on the transfer sheet or causing a discrepancy in colors in the image. In addition, since the conductive contacting member has a sheet-shape or roller-shape, it cannot be re-contacted with the dielectric sheet immediately after it passes through the trailing ends of the connecting portion and the grippers. Particularly, since it is difficult to attain proper contact between the trailing ends of the connecting portion and the grippers, and the dielectric sheet, the transferring efficiency is considerably worsened at the leading end of the transfer sheet, i.e., at the leading end of the image, thus causing a transfer void at the leading end of the image.